That is not the RULE for festivals. I've been to day festivals, with acts playing 1-2 hours sets, it was only a few acts and it was great (Mountain Jam in New York).

Also, weekend festivals (bonnaroo) do a good job of giving each artist a good amount of time to do their thing.

These, lets block out enough time for a regular concert and fit in 5 acts so we can call it a festival, shows are the problem. It's just them trying to overcharge and squeeze a little more money out of the fans.